The TCI algorithm is based on pharmaco-kinetic data derived from previous studies in a relatively small number of patients, by a "best fit" relationship between blood levels, infusion rates and other factors (such as age and weight). Several factors make it possible for a wide discrepancy between the predicted and the true blood concentrations. This difference can be higher in children compared to adults. Also blood loss and administration of large volumes of intravenous fluids can affect the blood concentrations. This study aims to identify the difference between the predicted and true blood concentrations by using Pelorus 1500, a bedside blood propofol measurement device, in children undergoing major spinal surgery under TIVA.